The present invention relates generally to woven yarn tubing or conduit used for protecting wiring harnesses and the like and, more particularly, to the application of a protective plastic coating to such tubing.
Tubing or conduit for enclosing and protecting wiring harnesses used, for example, in automobiles and electric appliances, is conventionally made by employing a circular loom to produce a woven yarn tubing base material. The woven yarn tubing is made in various sizes, normally ranging from 1/8 inch to one inch in diameter. While a variety of materials may be employed, commonly twisted paper is wound generally in a helix to form the woof yarn, and a cotton/polyester fiber blend forms the warp yarn, running generally lengthwise. The twisted paper woof yarn typically ranges in diameter from 0.030 inch to 0.070 inch, depending on the diameter of the tubing being made, while the cotton/polyester fiber warp yarn typically ranges from #14 cotton count to #8/3 cotton count, again depending on the diameter of the tubing being made. These yarn sizes result in woven yarn tubing with corresponding surface contours.
The woven yarn tubing base material is then waterproofed by impregnating the fibers with a latex sealant. Conventionally, an outer protective layer of asphalt is then formed. In particular, the asphalt is applied as a hot liquid which, upon cooling, immediately solidifies. Thereafter, the asphalt-coated tubing is cut to length and, in some cases, slit lengthwise.
The traditional asphalt-coated conduit, while relatively low in cost, and generally suitable, nevertheless leaves room for improvement.
Thus, alternative coating materials have been employed, such as plastic materials. Plastic as a coating material has a number of advantages compared to asphalt. Plastic is more resistant to oil. Plastic has better resistance to temperature and abrasion. Plastic-coated conduit is easier to cut and slit cleanly than asphalt-coated conduit. In addition, plastic affords the possibility of having a variety of colors, other than black.
Heretofore, plastic coatings have been applied to latex-impregnated woven yarn tubing by a process which involves pulling the tubing upwardly through a coating pot containing a liquid plastisol material and having a resilient circular exit die through which the tubing exits the coating pot vertically, forming a layer of liquid plastisol on the tubing. The plastisol layer is then cured by pulling the tubing further upwardly through a generally cylindrical gas-fired curing oven, having an annular burner near the bottom.
While plastic coatings to woven fabric tubing have previously been applied by the process summarized just above, due to a number of disadvantages such is typically employed only in special circumstances, for example where a particular color is desired.
One disadvantage of the prior process is its relatively high cost, perhaps twice as much as an asphalt coating process. One reason for the higher cost is that the coating process proceeds relatively slowly, typically in the order of less than two feet per minute, in sharp contrast to asphalt coating processes, which can proceed at rates of up to 325 feet per minute. Asphalt does not need to be cured; it solidifies to its final form simply upon cooling. Another reason for the relatively high cost of the prior plastic coating process is the cost of the plastisol material, which is significantly higher than the cost of asphalt coating material.
Another disadvantage of the prior process is that the resultant tubing is not uniformly circular in cross section. While the plastisol is uncured or in the process of curing, it is highly susceptible to deformation, which can become a permanent deformation in the finished product. Moreover, more plastisol coating material tends to be used when the tubing is not circular in cross section.